No that's fine....its all as clear as mud!......but that's not your
fault.
To recap...
"type" introduces a synonym for another type, no new type is
created....it's for readabilities sake.
"Newtype" introduces an isomorphic copy of an existing type...but
doesn't copy it's type class membership...the types are
disjoint/distinct but isomorphic (thus only 1 constructor param).
"data" introduces a new type, and defines a composition of existing
types to create a new one based on "->" and "(".
"class" introduces a constraint that any types declaring themselves to
be a member of this class...that functions must exist to satisfy the
constraint.
I'm sure that's wrong, but it's a good as I've got at the moment.
And to a degree it's all upside down....what Haskell thinks are
types...I think are "singnatures" and what Haskell thinks is a type
"class" I think of as a type.....it's not going to be easy.
-----Original Message-----
From: Thomas Davie [mailto:tom.davie at gmail.com]
Sent: 17 December 2007 12:35
To: Nicholls, Mark
Cc: Haskell Cafe
Subject: Re: [Haskell-cafe] OOP'er with (hopefully) trivial
questions.....
On 17 Dec 2007, at 12:22, Nicholls, Mark wrote:
> Ok...
>> Thanks I need to revisit data and newtype to work out what the
> difference is I think.
Beware in doing so -- type, and newtype are not the same either. type
creates a type synonim. That is, if I were to declare
type Jam = Int
then Jam and Int from that point on become completely interchangable,
the only thing this does is make things readable. For example, a
parser might be described as a function that takes a list of tokens,
and outputs a parse tree, and a list of unparsed tokens:
type Parser = [Token] -> (ParseTree, [Token])
if I write some parser combinators, I can now give them clear types like
<|> :: Parser -> Parser -> Parser
I could however still write this, and it would have *exactly* the same
meaning.
<|> :: ([Token] -> (ParseTree, [Token])) -> ([Token] -> (ParseTree,
[Token])) -> [Token] -> (ParseTree, [Token])
newtype on the other hand introduces a new type to the type system.
Because of this, the type system has to be able to tell when you're
using your new type, so a tag gets attached.
newtype Ham = Ham Int
This creates a type that contains only an integer, but is different
from Int (and Jam) in the type system's eyes. Thus, I cannot for
example write
(Ham 5) + (Ham 6)
Because Ham is not Int and thus (+) does not work (or actually, more
specifically, Ham is not a member of the class Num, the numeric types,
and therefore (+) doesn't work). This can of course be fixed thus:
newtype Ham = Ham Int deriving Num
Hope that helps
Tom Davie
p.s. Sorry for the slip with the newtype Rectangle.